The present invention relates generally to systems and methods for accurately and rapidly characterizing bridge sensors so they can be promptly used for design and/or development for various applications.
FIG. 1A shows a typical bridge sensor 49A wherein the temperature coefficient of a Wheatstone bridge sensor including bridge resistors 50, 51, 52 and 53 is used to develop a temperature-dependent voltage across a temperature-sensing resistor Rt. The voltage across temperature-sensing resistor Rt is referred to as the “temperature signal” Vtemp. An excitation voltage Vexc is applied to the junction 17 between resistors 50 and 51. Temperature-sensing resistor Rt is connected between ground and the junction 57 between resistors 52 and 53, and is considered to be connected at the “bottom” of bridge sensor 49A. A bridge sensor output signal Vsensorout+ is produced at the junction 55 between resistors 51 and 53, and another bridge sensor output signal Vsensorout− is produced at the junction 56 between resistors 50 and 52. The differential output of the bridge sensor is equal to Vsensorout+ minus Vsensorout−.
It should be understood that one or more of the resistors 50, 51, 52 and 53 is a transducer, the resistance of which is modulated by an applied stimulus, such as pressure, torque, humidity or the like. The modulated resistance of such a transducer results in a variation in the bridge sensor output signal.
FIG. 1B shows a another typical bridge sensor 49B wherein the temperature coefficient of the Wheatstone bridge sensor is used to develop a voltage across temperature-sensing resistor Rt. As in FIG. 1A, voltage across temperature-sensing resistor Rt is the “temperature signal” Vtemp. In FIG. 1B, an excitation voltage Vexc is applied to one terminal of temperature-sensing resistor Rt, the other terminal of which is connected to the junction 54 between resistors 50 and 51. Temperature-sensing resistor Rt is considered to be connected at the “top” of bridge sensor 49B. A sensor output signal Vsensorout+ is produced at the junction 55 between resistors 51 and 53, and another bridge sensor output voltage Vsensorout+ is produced at the junction 56 between resistors 50 and 52.
FIG. 1C shows a bridge sensor 49C in which the Wheatstone bridge section is in close thermal contact with a diode D through which a current I is forced by an external current source. The current I is forced into the anode of diode D, the cathode of which is connected to ground. The forward voltage across diode D is the “temperature signal” Vtemp. Excitation voltage Vexc is applied to the junction 17 between resistors 50 and 51. The bridge sensor output signal Vsensor-out(+) is produced at the junction 55 between resistors 51 and 53, and another sensor output signal Vsensorout+ is produced at the junction 56 between resistors 50 and 52.
It is desirable to emulate bridge sensors such as those of FIGS. 1A-C because large amounts of time are required to perform the needed calibration of the bridge sensor signal conditioning system that is required before the bridge sensor can be optimally used in designing a user system that requires bridge sensor input signals. Typically, a single calibration cycle for a bridge sensor in an environmental chamber/oven at a single temperature, including times required for the temperature to completely settle, can be approximately 8 hours. (It should be appreciated that the term “calibration” refers to the situation wherein the sensor is used in conjunction with development and/or programming of a “sensor signal conditioning system”. Sensor signal conditioning systems typically are programmable so as to cancel out both the temperature drift of the sensor and nonlinearity of the bridge sensor with respect to its applied stimulus. The term “characterization” refers to the process wherein measurements are made of the bridge sensor which exhibit both the nonlinearity of the bridge sensor with respect to its applied stimulus and the temperature drift of the bridge sensor.)
Typically, a customer may want to develop an application solution that needs to utilize the signals produced by a bridge sensor. Typically, the output signal produced by a bridge sensor is quite nonlinear with respect to a applied stimulus, such as pressure, torque, or other parameter. The bridge sensor output signal typically also varies or drifts considerably with respect to temperature. Also, bridge sensors often have non-repeatability or hysteresis problems.
A bridge sensor typically produces a low level output signal, e.g., in millivolts which is quite nonlinear with respect to the applied stimulus and which drifts over a temperature range. That small, nonlinear, temperature-dependent bridge sensor output signal often needs to be amplified and converted to a linear, temperature-independent, large-magnitude output signal in the range of, e.g., 0 to 5 volts. Typically, a sensor signal conditioning chip or system is utilized to solve these problems. However, sensor signal conditioning chips typically are complicated systems, and the user may need to learn to use software associated with the calibration of a bridge sensor or write software and/or develop hardware that involves a sensor signal conditioning chip and associated circuitry. The software referred to often is available for various sensor signal conditioning chips that are commercially available. The manufacturers of the various sensor signal conditioning chips typically either provide such software or provide instructions to customers on how to generate the software themselves.
Typically, the bridge sensor and the associated hardware must be placed in an environmental chamber/oven and thermally cycled to various temperatures at which the bridge sensor signal conditioning chip needs to be calibrated, and the proper coefficients need to be determined and stored into the sensor signal conditioning chip to compensate the various nonlinearities and drifts of the bridge sensor.
It should be appreciated that the amount of time required for cycling the temperature of the bridge sensor from cold to room temperature and from room temperature to hot in an environmental chamber/oven is the main factor that makes calibration/characterization of a bridge sensor very time-consuming. This is because it may require quite a few hours for the oven temperature, and hence the bridge sensor temperature, to fully stabilize. A typical three point calibration (−55 degrees C., 25 degrees C., and 85 degrees C.) for example, could require as much as eight or more hours to perform.
Another difficulty in working with bridge sensors is that in some cases costly specialized test equipment is required to provide the stimulus required to characterize the bridge sensors (e.g., stimulus such as humidity, acceleration, pH, pressure, strain, etc.). In many cases, access to such specialized test equipment is limited or unavailable. Furthermore, the equipment that provides the real world applied stimulus is sometimes custom developed for a specific bridge sensor and as such may be unproven.
The design/development of a system including bridge sensor signal conditioning electronics and its associated calibration software can be a difficult and time-consuming task. It would be highly desirable to have a device that could rapidly characterize bridge sensors and thereby substantially reduce the time required for the initial design stages in developing systems that utilize bridge sensors. For example, if a developer were to encounter non-repeatability errors while working on development of a sensor signal conditioning system while using a bridge sensor as the signal source, the developer would not know if the non-repeatability errors were caused by the bridge sensor, by the bridge sensor applied stimulus source (e.g. accelerometer, or pressure source), or by the bridge sensor signal conditioning system. In some cases, engineers have limited access to the hardware/software involved. In other cases this equipment is being developed in parallel with the sensor signal conditioning system and consequently is not available for use. It would be very advantageous to the developer to be able to have true picture of the accuracy and capability of a bridge sensor signal conditioning system apart from or independently of repeatability problems associated with the bridge sensor.
There is an unmet need for a system and method that greatly reduces the amount of time required for development of a sensor signal conditioning system.
There also is an unmet need for a system and method that avoids the large amount of time required for calibration cycles of bridge sensors during development of a sensor signal conditioning system.
There also is an unmet need for system and method that enables a developer to rapidly de-bug bridge sensor calibration software and hardware associated with sensor signal conditioning systems.
There also is an unmet need for a system and method that can eliminate non-repeatability problems of bridge sensors during the development of an associated sensor signal conditioning system.
There also is an unmet need for a system and method which can allow a developer to get a true picture of the accuracy and capability of a bridge sensor signal conditioning system without directly utilizing a bridge sensor to allow the developer to focus on issues with the sensor signal conditioning system separately or independently from problems with the bridge sensor.